364 Refra&ion of Heat. 



" it is be noticed that our proof goes only to the power of occafioning heat, which has been 

 " ftri£lly afcertained by the thermometer. If it ftiould be faid that the power of illuminating 

 " objefts, of thefe fame rays, is as fl;rl£l:ly proved by the fame experiments, I muft remark 

 " that from the cafes of invifible rays brought forward in the four lad experiments, it is 

 " evident that the conclufion that rays mud have illuminating power, becaufe they have a 

 *' power of occafioning heat, is erroneous j and, as this muft be admitted, we have a right 

 " to aflc for fome proof of the aflertion, that rays which occafion heat can ever become vi- 

 " fible. But, as we Ihall have an opportunity to fay more of this hereafter, I proceed now 

 " to inveftigate the refradion of heat-making rays." 



Experiment It. RefraBicn of folar heat. The fun's Jight was received in a new ten feet 

 Newtonian telefcope; the mirror of which has 24 inches in diameter of polilhed furface. 

 The rays were made to pafs through a day-piece with four lenfes, and thence to the ball of the 

 thermometer placed in their focus. As foon as the rays were brought to the thermometer 

 it rofe almoft inftantly through 70°, when the telefcope was turned away to avoid the dan- 

 ger of cracking the glafles. Here the rays which occafioned this fudden augmentation 

 of heat had undergone eight fuccefllve refradtions, fo that their being fubjedl to its laws 

 cannot be doubted. 



Experiment 12. RefraElion of the heat of a candle. A lens of about 1,4 inch focus, and 

 I.I diameter, mounted on a fmall fupport was placed at the diftance of 2.8 inches from a 

 candle, and a thermometer was very carefully placed in the fecondary focus of the candle 

 behind the lens. Not far from the lens towards the candle was a pafteboard fcreen with an 

 aperture of nearly the fame fize as the lens ; and the lens itfelf was fupported on an ec- 

 centric pivot, fo that it could be turned away from its place and reftored at pleafure. This 

 arrangement being made, the thermometer was for a few moments expofed to the rays of 

 the candle till it had aflumed the temperature of its fituation. Then the lens was turned 

 on its pivot, fo as to intercept the direft rays which pafled through the opening in the pafte- 

 board fcreen, and to refradl them to the focus in which the thermometer was placed. In 

 three minutes the thermometer received 2^ degrees of heat by the refra£lion of the lens, 

 which it again loft in the fame time when the lens was turned away, and in three minutes 

 more the fame increafe of heat was produced by replacing the lens in its fituation. A greater 

 efFeft could be produced by a different arragement of the diftances. Thus when the lens 

 was placed at 3^- inches diftance from a wax candle, the thermometer in the fecondary 

 focus was raifed from 5 to 8 degrees, according to the burning of the candle and the accu- 

 racy of the adjuftment. 



Experiment 1 3. Refraction of the heat that accompanies the coloured part of the prifmatic fpeC' 

 irum. The coloured part of the prifmatic folar fpe6lrum being admitted through a perfo- 

 rated fcreen of pafteboard, upon a burning glafs, raifed the tenaperature of a thermometer 

 1 12 degrees in one miniue. 



Experiment 



